It is well known that when welding work is being conducted, intensive radiation is emitted from a welding arc as well as from the melting zone of the materials being worked on. Such radiation is of an intensity which is harmful to the human eye. At present, in order to protect the operator from this harmful radiation, protective goggles or a protective shield are used which comprise a central region of spectrally absorbing glass which filters out much of the radiation. Moreover, these glasses have the effect of attenuating the intensity of the radiation to values which are harmless to the eye. Unfortunately, these glasses greatly reduce the visibility of the surroundings to the operator so that before the arc is lit and welding commences, the operator or welder can see very litte. As a result, the ability of orientation of the welder will be unsatisfactory and he will therefore have to remove or push aside the shield or the goggles in order to place the welding electrode in its correct position at the start of welding.
To improve conditions for the welder, it has been suggested to use an interference filter instead of the aforementioned spectrally absorbing glass. The filter would have pass bands for one or more ranges of wavelengths in the visible part of the spectrum of the arc, the intensity in these ranges of wavelengths having values which are harmless to the eye. The remaining wavelengths in the spectrum are extinguished by the filter. However, the use of an interference filter of these properties also has the disadvantage that the visibility of the surroundings is unsatisfactory.
One method to achieve visibility of the surroundings, even when the welding arc is not lit, consists in the use of a special light source in the surrounding space. The spectrum of the light source will be restricted to one or more narrow ranges of wavelengths. Sodium lamps are an example of such a light source. The interference filter which is used in the protective glass will then have pass bands for the range or ranges of wavelengths emitted by the light source. With the help of such a light source in the surrounding space visibility of the surroundings is obtained both before the arc has been lit and while it is lit.
This arrangement, however, has a substantial disadvantage in that the radiation from that part of the spectrum of the arc which lies outside the pass band of the filter will be strongly reflected from the surface of the interference filter and consequently disturb the surroundings. This mirror-like strong reflection from the filter also has the disadvantage that, when the welding arc is not lit, the welder will see his mirror image in the protective glass instead of the object he wants to observe through the protective glass.
Mirror-like qualities of the interference filter can be overcome by putting optically thick layers of absorbing material on both sides of the interference filter. The absorbing material on the front side would prevent the mirror image from disturbing the surrounding areas while the absorbing material on the other side would prevent the operator from seeing his reflection. Unfortunately, the placing of optically thick absorbing material on both sides of the interference filter would result in a device which was bulky and therefore not suited as lenses for a welding mask and further would be too expensive to manufacture commercially.
Since interference filters operate upon principles involving the thickness of the layers thereof, the adding of thin absorbing layers to the front and back of the interference filter would be expected to change the spectral characteristics thereof producing a new interference filter which would still have the mirror-like characteristics.